(1) Field of the Invention
The present invention relates to a processing aid for a thermoplastic resin, which is obtained by polymerizing a monomer or monomer mixture composed mainly of methyl methacrylate in the presence of a polymer composed mainly of a methacrylic acid ester other than methyl methacrylate. More particularly, the present invention relates to a processing aid having properties such that, when this processing aid is incorporated into a thermoplastic resin such as a vinyl chloride resin, a polycarbonate resin, a polyester resin, an ABS resin, a styrene resin, a methacrylic resin or a polyethylene resin, an excellent gelation-promoting effect is obtained without degradation of the inherent characteristics of the thermoplastic resin, and since the viscosity in the molten state is not increased, the formation of air marks or flow marks is controlled, and to a thermoplastic resin composition comprising this processing aid.
(2) Description of the Related Art
Since thermoplastic resins, especially vinyl chloride resins, have excellent physical and chemical properties, they are widely used in various fields, but since the processing-possible temperature is close to the heat decomposition temperature in vinyl chloride resins, they have problems in that the processing-possible temperature range is extremely narrow and the processability is poor.
To solve this problem, a process has been adopted in which a plasticizer is incorporated in a vinyl chloride resin. According to this process, the above-mentioned problem is solved to a certain extent, but since the incorporated plasticizer is volatilized at the heat shaping step, the intended object cannot be attained, and the mechanical properties of the obtained shaped article are degraded.
To improve the processability, that is to increase the gelation speed at the step of processing and shaping a vinyl chloride resin, render deep-draw-forming possible, maintain an unchanged gloss on the surface of a shaped article even if processing and shaping are carried out for a long time, or smooth the surface of the obtained shaped article, a process has been proposed in which a copolymer composed mainly of methyl methacrylate is incorporated as a processing aid (see, for example, Japanese Examined Patent Publication No. 49-19095, Japanese Examined Patent Publication No. 52-1746 and Japanese Examined Patent Publication No. 53-2898).
In a vinyl chloride resin composition comprising a processing aid composed mainly of the above-mentioned specific copolymer, the gelation speed is high at the processing step and the tensile elongation at a high temperature is increased, and therefore, deep-draw-forming becomes possible and an application of vacuum forming or contour extrusion also is possible. The vinyl chloride resin composition disclosed in the above-mentioned Japanese Examined Patent Publication No. 52-1746 can be used in not only the rigid resin field but also the soft resin field, because the formation of fish eyes is controlled.
Nevertheless, a resin composition formed by incorporating a processing aid as mentioned above inherently has a high stickiness to a metal surface and has a high melt viscosity, and therefore, the torque (resistance to kneading) is greatly increased at the mold-processing step and the generation of heat is extreme. Namely, the resin composition is defective from the productivity viewpoint. Moreover, when the resin composition is shaped into a sheet or film, flow marks and air marks are formed, and the commercial value of the shaped article is degraded. The use of various lubricants in combination with this processing aid has been attempted as means for overcoming these defects, but to retain the physical properties of the vinyl chloride resin, the amount of the lubricant used is limited, and this method is not a general solution. Although the torque at the kneading step can be reduced by an addition of a plasticizer or the like, the inherent mechanical properties of the thermoplastic resin are degraded and a new problem, for example, a formation of unmolten parts (fish eyes), arises, and satisfactory solution cannot be obtained.